An eddy current test probe using an oscillator mounted in a digitally manipulatable housing

ABSTRACT

A device for eddy current testing of metals for cracks and the like including a sensing coil disposed in a contoured contact tip carried at one end of a hollow, cylindrical housing which houses electrical circuitry and a battery for powering the device. The electrical circuit is closed by plugging an earphone into a jack in the side of the housing, and the circuitry includes a blocking oscillator connected by a center tap to the sensing coil so that this single coil forms a portion of the tuned circuit of the oscillator. The blocking frequency developed by the circuit is in the audible range and is adjusted for purposes of metals testing by the use of a potentiometer connected in the control circuit. A change of the developed blocking frequency, as evidenced by a change in the signal heard at the earphone, is indicative of the presence of an anomaly in the metal under test.

United States Patent m1 Samples [54] EDDY CURRENT TEST PROBE USING ANOSCILLATOR MOUNTED IN A DIGITALLY MANIPULATABLE HOUSING [76] Inventor:Everett L. Samples, 2916 S. W. 60th St., Oklahoma City, Okla.

221 Filed: Sept. 28, 1970 211 Appl.No.: 75,977

UNITED STATES PATENTS 2,993,167 7/1961 Smith ..324/72.5 3,170,113 2/1965Harmon ..324/4O 2,321,356 6/1943 Berman..... ....324/41 3,546,62812/1970 Zitter ..324/3 FOREIGN PATENTS OR APPLICATIONS France ..324/3Great Britain ..324/37 [4 1 Mar. 27, 1973 OTHER PUBLICATIONS Yanczer,P.; Blocking Oscillator Gives HighSpeed Linear Ramp; Electronic Design4, Feb 15, 1968; p. 122.

Primary ExaminerRobert J. Corcoran Attorney-Dunlap, Laney, Hessin &Dougherty [57] ABSTRACT A device for eddy current testing of metals forcracks and the like including a sensing coil disposed in a contouredcontact tip carried at one end of a hollow, cylindrical housing whichhouses electrical circuitry and a battery for powering the device. Theelectrical circuit is closed by plugging an earphone into a jack in theside of the housing, and the circuitry includes a blocking oscillatorconnected by a center tap to the sensing coil so that this single coilforms a portion of the tuned circuit of the oscillator. The blockingfrequency developed by the circuit is in the audible range and isadjusted for purposes of metals testing by the use of a potentiometerconnected in the control circuit. A change of the developed blockingfrequency, as evidenced by a change in the signal heard at the earphone,is indicative of the presence of an anomaly in the metal under test.

7 Claims, 3 Drawing Figures EDDY CURRENT TEST PROBE USING AN OSCILLATORMOUNTED IN A DIGITALLY MANIPULATABLE HOUSING BACKGROUND OF THE INVENTION1 Field of the Invention This invention relates to metal testingdevices, and more frequently, to devices for detecting anomalies in ametal structure, using the effect of such anomalies on eddy currentsdeveloped in the metal structure for providing a signal in an oscillatorcircuit contained within the device to provide a readout or indicatioverifying the presence of the anomaly.

2. Brief Description of the Prior Art In the field of non-destructivetestin'gof metals, both of the ferrous and non-ferrous types, eddycurrent detector devices have previously been utilized with considerablesuccess. These devices, in general, utilize a detector coil which isplaced in inductive relation to the metal part to be tested. Thedetector coil is excited by alternating current developed by anoscillator circuit. The amplitude or frequency of the output signal ofthe oscillator circuit is, in some types of eddy current detectordevices, varied as the detector coil is placed in proximity to ananomaly in the metal. This is due tothe anomaly causing fluctuations inthe eddy currents present in the metal.

A number of types of eddy current detector devices of the type describedhave been heretofore pgoposed with various types of oscillator circuitsbeing coupled to the sensing or detector coil, and various arrangementsproposed for mounting the coil in the tip of a probe. These latterarrangements often seek to minimize the effect on the frequency of theoscillator circuit of variations in the distance of the probe from thetest piece, or of the presence of coatings of paint and the like so thattrue signal variations due to the presence of cracks or other flawssought to be detected are more accurately discerned. Difficulty has beenencountered in many instances in eliminating spurious signals due tosurface roughness of the metal under test, presence of oxide coatings,dirt, and the like. In some of the eddy current probe type instrumentspresently on the market, the surface of the metal must be thoroughlycleaned and smoothed before testing if any degree of accuracy is to beobtained. Other probe instruments require a jig to maintain the probe ina precise predetermined position with respect to the test piece duringthe testing, and a sensitive and rather large meter is used to providean indication of the presence of cracks and other anomalies in themetal.

Three types of eddy current testing devices in which a sensing ordetector coil is coupled to an oscillator circuit and depend in theirusage upon the frequency ofa signal developed thereby are disclosed inU. S. Pat. No. 2,939,073, U. S. Pat. No. 2,581,394, and U. S. Pat. No.3,135,914. In each of these systems, however, as with substantially alleddy current testing devices now in use, the circuitry employed iscomplex, and requires considerable training and experience on the partof the operator in order to successfully operate the devices to make anaccurate determination of the presence of cracks and other flaws in themetal under test. Moreover, many of the devices are very bulky and aredifficult to transport from one location to the other. In someinstances, the metal to be tested may be more conveniently brought tothe sit'us of the test device than the device may be transported to thelocation of metal members to be subjected to testing.

In a currently widely used procedure for testing aircraft and otherlarge metal structures for the presence of cracks or weakened zones,initial testing is frequently carried out by first inspecting theequipment visually by the use of a flashlight and inagnifying glass.Such inspections frequently require the removal of paint or othersurface coating in order to sufficiently verify the presence of a crackor .other anomaly to indicate the desirability of further and more exactand comprehensive testing. At times, the structure under test must bedisassembled to permit the part in which the flaw is located to beremoved to a laboratory for non-destructive testingwith sophisticatededdy current testing equipment. A real need has therefore existed for avery small portable testing device which can quickly and reliablyindicate to the inspector in on-the-site inspections, the presence ofany cracks or anomalies without the necessity of paint removal, andwithout the necessity of further more specific and exacting testingexcept in rare instances. For tests of this type, an instrument of theneeded type also would necessarily be simple in its operation and easilyused by an inspector having a minimal knowledge of electronics andlittle training in the use of the instrument.

BRIEF DESCRIPTION OF THE PRESENT INVENTION The present invention is aportable eddy current test instrument capable of non-destructivelyinspecting metallic materials for surface, or slightly subsurface,defects and anomalies. The device may also be used for evaluatingcoating thicknesses, test piece conductivity, test piece permeabilityand material structural integrity by comparative testing with knownstandards.

Broadly described, the testing instrument of the invention comprises asmall portable housing of generally cylindrical configuration having agenerally conical probe or contact tip located at one end thereof.Within the probe, a sensing or detector coil is located, and this coilis center tapped and connected in parallel with a capacitor to form theinductive element in a tuned circuit comprising a portion of a blockingoscillator carried within the cylindrical housing. The blockingoscillator is powered by a small battery also carried within the housingat another location, with such blocking oscillator including atransistor powered by the battery and a feedback circuit for providingregenerative feedback to the transistor. A blocking frequency isdeveloped by the circuitry, with control of this frequency provided by apotentiometer connected in the basepower supply circuit of thetransistor.

The blocking oscillator circuitry is such that the detector coil reactsinductively to the eddy current developed in a metal test piece underexamination, and concurrently provides an effective circuit element inthe oscillator effecting both the normal frequency developed in thedetector coil, and the blocking frequency developed by the oscillatorwhich is used to actuate an earphone connected in the emitter circuit ofthe transistor. The earphone is arranged within the circuitry so thatits connection in the circuit by inserting a plug-in jack operates toclose the circuit and make it immediately operative. A variation in theblocking frequency is indicated by a change in the tone developed in theearphone, and such tone change provides the necessary audible indicationof the presence ofa flaw or anomaly in the metal under test.

From the foregoing description of the invention, it may be perceivedthat the present invention provides a compact, portable test instrumentwhich may be utilized for non-destructive testing of metals utilizingthe eddy current principle.

Another object of the invention is to provide an eddy current detectordevice which may be manually utilized by an inspector with littletraining and experience in the use of the instrument to determine, bythe development of an audible signal, the presence of surface andsubsurface defects in a metal specimen under test.

A further object of the invention is to provide a compact and portableeddy current detector device in which an earphone utilized to audiblyindicate the presence of flaws or anomalies in a metal under test alsofunctions as an ON-OFF switch for energizing and de-energizing theinstrument so that the instrument is not inadvertently permitted toremain powered at times when it is not in use.

A further object of the invention is to provide an eddy current testinginstrument utilizing a blocking oscillator circuit which permits asignal of more desirable, relatively high frequency to be passed throughthe detector coil used in the device, and an audible signal of lowerfrequency to be developed at an earphone included in another portion ofthe circuit.

A further object of the invention is to provide a testing technique fornon-destructive testing of metals which effectively supplements visualinspection constituted by illumination of the metal under test, andexamination ofthe metal under magnification.

Another object of the invention is to provide a small portable eddycurrent testing device for non-destructively testing metals, whichdevice is battery powered by a battery of relatively small E.M.F. whichis quickly replaceable in the device, and which powers a miniaturized,transistorized blocking oscillator circuit during the operation of thedevice.

Additional objects and advantages of the invention will become apparentas the following detailed description of the invention is read inconjunction with the accompanying drawings which illustrate theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of theeddy current testing device of the invention.

FIG. 2 is a side elevation view of the eddy current testing device withparts broken away and shown in section to illustrate certain circuitelements located within the housing of the testing device.

FIG. 3 is a circuit diagram depicting the electronic circuit utilized inthe testing device of the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTIONReferring initially to FIG. 1 of the drawings, shown therein is anelongated cylindrical housing which has a frusto-conically shaped probeor contact element 12 mounted on one end thereof, and a closure cap 14detachably retained on the other end by screws or other suitable means.The cylindrical housing 10 is partitioned by a plug 16 so that thehousing is divided into a battery chamber 18 and a circuitry chamber 20.Opening into one side of the housing 10 is ajack socket 22 for receivingthe plug or jack 24 of an earphone as hereinafter described. There isalso provided in one side of the housing 10 an opening through which ismounted a shaft (not visible) connected at one of its ends to afrequency control knob 26 which functions in a manner hereinafterdescribed.

Access to the circuitry contained within the circuitry chamber 20 is hadby removal of a plate 28 forming a portion of the side wall of thehousing 10, and detachably retained in position by screws or othersuitable means. In FIG. 2 of the drawings, the plate 28 is shown removedfrom the housing 10 and a portion of the housing adjacent the plate isbroken away to show the battery chamber 18 containing a battery 30 forpowering the instrument. The battery 30 is inserted in the batterychamber 18 through the open end of the housing 10 when the closure cap14 has been removed. When the battery is positioned in the batterychamber 18, one end thereof bears against a spring contact 32 which isconnected through the partition plug 16 to a portion of the circuitrydisposed in the circuitry chamber 20. The opposite end of the battery isconnected to a suitable electrical lead which passes along the wall ofthe housing 10 and also passes through the partition plug 16 forconnection to the circuitry.

The conical probe 12 secured to one end of the housing 10 has a hollowinterior and a specially shaped exterior which provides for uniformdistribution of the lines of flux around the tip of the probe ashereinafter described. The tip of the probe 12 is rounded. Positionedinside this tip is a sensing or detector coil 34. The detector coil 34includes a series of turns or convolutions of wire disposed in a bore inthe tip of the probe 12 and surrounding a ferrite core 38. Leads extendfrom the coil 34 through the enlarged portion of the probe, and thisportion of the probe is filled with a suitable potting material such asepoxy cement or the like.

The circuitry utilized in the eddy current testing instrument of thepresent invention is depicted in FIG. 3 of the drawings. The circuitryutilized is relatively simple as contrasted with other eddy currenttesting devices heretofore in use, and constitutes a blocking oscillatorcircuit which includes the detector coil 34. The coil 34 is centertapped at 40, with a portion of the detector coil connected in parallelwith a capacitor 42 to provide a tuned circuit. Although a fixed valuecapacitor is illustrated in the drawing, a variable capacitor can beemployed for adjusting the resonant frequency of the tuned circuit, andadjusting the frequency of the signal passed through the detector coil34.

From the tuned circuit constituted by the capacitor 42 and coil 34connected in parallel, an electrical lead 44 is extended to thecollector of a common emitter transistor 46. Another lead 48 passes fromthe tuned circuit to the battery 30 used to power the circuit. TheE.M.F. developed by the battery 30 may be relatively small and mayrange, for example, in various embodiments of the device, from about 1.5volts to about 9 volts.

Connected between the battery 30 and the emitter of the transistor 46 isan earphone 50 which is connected into the circuit by plugging the jack24 into the jack socket 22. In other words, until the jack 24 is pluggedinto the jack socket 22, the circuit of the instrument is open and nodissipation of power from the battery occurs. A current limiting circuitwhich is used to shape the pulses applied to the earphone 50 during theuse of the device is connected between the earphone 50 and the emitterof the transistor 46, and includes a fixed resistance 52 and a capacitor54. Regenerative feedback for the operation of the blocking oscillatoris developed through the lead 56 which is connected to one end of thedetector coil 34 and which contains a capacitor 58. The opposite end ofthe lead 56 is connected to the base of the transistor 46. The basecircuit of the transistor contains a trimming resistor 60 connected in alead 62 and also contains a potentiometer 64 which is connected to thepositive side of the battery 30. The potentiometer 64 is utilized forvarying the blocking frequency developed by the circuit, and adjustingthe signal to the earphone 50 as hereinafter explained. The movable tapof this potentiometer is connected to the shaft which is connected atone end to the control knob 26.

OPERATION In the operation of the eddy current testing instrument of theinvention, the instrument is used most frequently to supplement visualinspection of metallic materials for defects or for anomalies in themetallic test specimen. In this usage, an area of the metallic partunder observation will have been considered suspect because of visualindications such as cracks in coatings, scratches or the like, orbecause of some previous failure or defect patterns which have developedin the metal. Such symptoms of cracking or other defects are determinedvisually by inspection with a flashlight and/or a magnifying glass.

After locating such a suspect area, the inspector may then, withoutcalling for special equipment, removal of the metal article to alaboratory, or the necessity for removing paint or other surfacecoatings, proceed to use the eddy current testing device of the presentinvention. Initially, the earphone 50 is connected into the blockingoscillator circuitry of the instrument by placing the jack 24 in thejack plug 22. This closes the circuits so that current can flow in thecircuit from the battery 30. The probe 12 is then placed against themetal in the suspect area with the longitudinal axis of the cylindricalhousing extending substantially normal to the metal under test. In thisconnection, it is pointed out that the particular contour and geometricconfiguration of the probe, and particularly the rounded tip of theprobe, provides an equalized flux density around this portion of theprobe so that slight variations in the angulation of the instrument withrespect to the surface of the metal under test do not result in spurioussignals being developed by the instrument as a result of non-uniformityof the flux density around the coil 34 located in the tip of the probe.

With the test instrument in position with the tip of the probe againstthe metal, the control knob 26 is then rotated by the inspector so thatthe setting on the potentiometer 64 is varied. This has the result ofvarying the value of the blocking frequency actuating the earphone 50 soas to change the tone heard in the earphone. The control knob 26 isrotated until the tone heard in the earphone 50 by the inspector fadesoutthat is, becomes inaudible. Then the knob is rotated very slightlyuntil the exact point in the rotation of the control knob is reachedthat the tone is recaptured or is again heard.

It may be noted that at this point in the procedure, a slight variationin procedure is involved in the case of the testing of ferrous metals ascontrasted with non-ferrous metals. When ferrous materials are undertest, the control knob 26 is set at the point at which the highestaudible frequency occurs, and the tone persists in the earphone. Withnon-ferrous metals, on the other hand, the control knob is movedslightly past this point to the first point at which no tone continuesto be heard.

With the instrument thus appropriately set, the probe 12 is then movedacross the surface of the metal with the point of the probe in contactwith the metal as it is moved. As the probe 12 moves across the metaldue to digital manipulation of the housing 10, any defects present inthe metal will be indicated by a loss or return of the tone heard in theearphone, depending upon whether ferrous or non-ferrous metals are beingtested. In other words, in the case of ferrous metals, the tone whichhas been heard up to the location where adefect occurs will be lost asthe tip of the probe passes over the crack or other defect in the metal.The opposite occurs in the case of non-ferrous metals and the tone isrecaptured in this case as the defect is located. This procedure iscontinued until the entire metallic specimen is tested. At the end ofthe test, the earphone 50 is unplugged from the jack socket 22 toautomatically disconnect the battery from the remainder of the circuitand prevent the power source from being dissipated prior to the nexttest.

Defect indication should always be rechecked for repeatability, and thevisible causes for any reactions indicated by a change of tone in theearphone should be taken into account before making finaldeterminations. Scratches or gouges, if relatively smooth, and if onlyoccurring in the paint or other coating present on the metal, will notproduce spurious signals indicative of cracks.

The principles upon which the present invention operates are, ingeneral, well known in the art of nondestructive testing of metals. Thedetector coil 34 mounted in the tip of the probe 12 is designed andmounted in a manner such that it will provide a consistentcoil-to-specimen relationship. Eddy currents induced in the metallicspecimen by the coils inductive flux react back through the flux toestablish an inductive reactance in the oscillator circuit, and thereby,produce a characteristic basic frequency for that particular metallicmaterial under test, and for that particular test configuration. Thedisruption of the eddy currents occurring in the test specimen andproduced by cracks or other structural anomalies in the metal result inchanges in the coils inductive reactance, thus causing a change offrequency in the oscillator circuit.

In the case of the blocking oscillator used in the present invention,the change in the resonant frequency of the tank circuit formed by thecapacitor 42 and a portion of the detector coil 34 effects the blockingfrequency developed in another portion of the oscillator circuit andpassed to the earphone 50. The blocking frequency is also determined bythe setting of the potentiometer 64 and the effective value of thevariable trimming resistor 60. These circuit elements are used toinitially set the instrument so that the fade-out of the tone heard inthe earphone, or near fade-out, are the set points of the instrumentupon commencement of the test. The set value of the blocking frequencyis then altered enough to make the tone fade out or be recaptured by thedevelopment of such alteration due to changes in the resonant frequencyof the tank circuit as the inductive reactance of the coil 34 is changedwhen cracks or other anomalies are encountered.

It will be apparent, of course, that the frequency range employed in thetesting, and characterizing the signal passed through the detector coil34 is determined by the values of the capacitor 42 and that portion ofthe coil 34 which is connected in parallel with this capacitor. Thisfrequency, which is termed the normal frequency, is substantially higherthan the blocking audio frequency developed at the earphone S0, and thispermits the advantage of obtainment of maximum flux saturation of thedetector coil 34, and maximum eddy current generation in the metalicspecimen under test. In a typical testing device such as the embodimentof the invention illustrated in the drawing, a normal frequency in thearea of 350 kilohertz has been utilized, and a blocking frequency ofabout 1700 hertz has been employed. However, other values may besatisfactorily used, and the particular application of the device willdetermine the values of the normal and blocking frequencies which areemployed.

The eddy current test instrument of the present invention offers manyadvantages over eddy current testing instruments heretofore in use. Itis, of course, a test instrument designed for non-destructivelyinspecting metallic materials for surface or slightly subsurface defectsaccording to principles now well known and understood. The instrument ofthe present invention is, however, a portable device having the powersupply and electronic circuitry contained in a tubular probe assemblyslightly larger than a standard pencil-type, two cell flashlight. Thetest signal is relatively easily understood and interpreted, consistingbasically of a simple go-no go indication. The device does not normallyrequire paint or coating removal in order to conduct an inspection, andmay be effectively used to detect heat damaged areas and also for thepurpose of separating and sorting metallic materials by comparativetesting. For example, the device may be used to separate aluminum foilsin 0.001 inch increments based upon differences in the location of thefade-out point of the tone heard in the earphone.

Although a preferred embodiment of the invention has been hereindescribed, it is to be understood that various changes and modificationsin the illustrated circuitry may be employed without departure from thebasic principles of the invention. Changes and innovations of this typeare therefore deemed to be circumscribed by the spirit and scope of theinvention, except as the same may be necessarily limited by the appendedclaims or reasonable equivalents thereof.

What is claimed is:

1. An instrument for non-destructively testing metal comprising:

a transistorized blocking oscillator including:

a tuned tank circuit including a center tapped detector coil and acapacitor providing parallel inductive and capacitive reactance;

a regenerative feedback circuit; and

a control circuit including a battery, a transistor, a potentiometer,and a variable trimming resistor for varying the blocking frequency ofsaid blocking oscillator;

a housing containing said battery, said feedback circuit, said controlcircuit, and a portion of said tank circuit, said housing having anearphone jack opening in one side thereof and connected in said controlcircuit for closing said control circuit when an earphone is pluggedinto said jack; and

a probe mounted on one end of said housing and containing said detectorcoil, said probe being a generally conically shaped contact memberhaving a rounded tip and having said coil positioned in immediatejuxtaposition to said tip.

2. An instrument for non-destructively testing metal as defined in claim1 and further characterized as including an earphone plugged into saidjack and closing said control circuit for responding to the blockingfrequency of said oscillator.

3. An instrument as defined in claim 1 wherein said center tapped coilis disposed in convolutions around the axis of said contact member in abore in said contact member provided therefor, and further including asoft iron core disposed concentrically in said coil and extending into acounterbore extending from said bore toward said rounded tip along theaxis of said contact member.

4. An instrument as defined in claim 1 and further characterized toinclude:

a rotatable blocking frequency selector knob located outside saidhousing; and

a shaft connected at one end to said frequency selector knob andextending through said housing for connection at its other end to themovable tap of said potentiometer.

5. An instrument as defined in claim 1 and further characterized toinclude a pulse shaping circuit comprising a resistor and capacitorconnected in parallel between said earphone jack opening and the emitterof said transistor in the emitter circuit of the transistor for limitingcurrent flow in said emitter circuit and shaping the pulses of theblocking frequency developed by said blocking oscillator.

6. In an eddy current testing instrument of the type including adetector coil for responding to fluctuations in eddy currents induced ina metallic test specimen, and an oscillator circuit developing an outputfrequency varying with the inductance of said detector coil, theimprovement which comprises:

a cylindrical, digitally manipulatable housing containing saidoscillator, and having an earphone jack in one side thereof, said jackbeing connected in said oscillator circuitry to provide switch contactscloseable upon plugging an earphone into said jack;

a battery disposed in said housing and connected in said oscillatorcircuit for powering said oscillator circuit;

a frequency varying element disposed in said oscillator circuit forvarying the output frequency thereof;

said frequency varying element comprising a variable resistance, and acapacitor cooperating to provide selective variation of the outputfrequency of said oscillator circuit; and

a digitally manipulatable frequency selector knob rotatably mounted onthe outside of said housing and connected through said housing to saidfrequency varying element to facilitate selective variation of theoutput frequency of said oscillator circuit; and

a tapered probe mounted on one end of said housing and adapted forcontainment of a detector coil in the tip of said probe; and

an iron core detector coil mounted in said tapered probe immediatelyadjacent the tip thereof.

7. An eddy current testing circuit comprising:

a battery source of power;

a common emitter transistor connected to said battery;

an iron core detector coil arranged to be disposed in inductiverelationship to metal to be tested;

a capacitor connected in parallel to a first portion of said detectorcoil through a center tap to form a tuned circuit developing a normaltesting frequen- Y;

a direct collector-to-base feedback circuit connected between saidcenter tap of said coil and the base of the transistor and including, inseries, a second capacitor and a second portion of said detector coil; 4

means included in the base circuit of said transistor for determiningwith said second capacitor, the value of a blocking audio frequencydeveloped in said testing circuit, said means including:

a potentiometer; and

a trimming resistor connected in series with said potentiometer;

an earphone connected in the emitter circuit of said transistor andresponsive to said blocking audio frequency to emit an audible signalindicative of changes in the inductive reactance of said iron coredetector coil; and

a pulse shaping circuit comprising a resistor and capacitor connected inparallel between said earphone and the emitter of said transistor in theemitter circuit of the transistor for limiting current flow in saidemitter circuit and shaping the pulses of the blocking frequencydeveloped by said blocking oscillator.

1. An instrument for non-destructively testing metal comprising: atransistorized blocking oscillator including: a tuned tank circuitincluding a center tapped detector coil and a capacitor providingparallel inductive and capacitive reactance; a regenerative feedbackcircuit; and a control circuit including a battery, a transistor, apotentiometer, and a variable trimming resistor for varying the blockingfrequency of said blocking oscillator; a housing containing saidbattery, said feedback circuit, said control circuit, and a portion ofsaid tank circuit, said housing having an earphone jack opening in oneside thereof and connected in said control circuit for closing saidcontrol circuit when an earphone is plugged into said jack; and a probemounted on one end of said housing and containing said detector coil,said probe being a generally conically shaped contact member having arounded tip and having said coil positioned in immediate juxtapositionto said tip.
 2. An instrument for non-destructively testing metal asdefined in claim 1 and further characterized as including an earphoneplugged into said jack and closing said control circuit for respondingto the blocking frequency of said oscillator.
 3. An instrument asdefined in claim 1 wherein said center tapped coil is disposed inconvolutions around the axis of said contact member in a bore in saidcontact member provided therefor, and further including a soft iron coredisposed concentrically in said coil and extending into a counterboreextending from said bore toward said rounded tip along the axis of saidcontact member.
 4. An instrument as defined in claim 1 and furthercharacterized to include: a rotatable blocking frequency selector knoblocated outside said housing; and a shaft connected at one end to saidfrequency selector knob and extending through said housing forconnection at its other end to the movable tap of said potentiometer. 5.An instrument as defined in claim 1 and further characterized to includea pulse shaping circuit comprising a resistor and capacitor connected inparallel between said earphone jack opening and the emitter of saidtransistor in the emitter circuit of the transistor for limiting currentflow in said emitter circuit and shaping the pulses of the blockingfrequency developed by said blocking oscillator.
 6. In an eddy currenttesting instrument of the type including a detector coil for respondingto fluctuations in eddy currents induced in a metallic test specimen,and an oscillator circuit developing an output frequency varying withthe inductance of said detector coil, the improvement which comprises: acylindrical, digitally manipulatable housing containing said oscillator,and having an earphone jack in one side thereof, said jack beingconnected in said oscillator circuitry to provide switch contactscloseable upon plugging an earphone into said jack; a battery disposedin said housing and connected in said oscillator circuit for poweringsaid oscillator circuit; a frequency varying element disposed in saidoscillator circuit for varying the output frequency thereof; saidfrequency varying element comprising a variable resistance, and acapacitor cooperating to provide selective variation of the outputfrequency of said oscillator circuit; and a digitally manipulatablefrequency selector knob rotatably mounted on the outside of said housingand connected through said housing to said frequency varying element tofacilitate selective variation of the output frequency of saidoscillator circuit; and a tapered probe mounted on one end of saidhousing and adapted for containment of a detector coil in the tip ofsaid probe; and an iron core detector coil mounted in said tapered probeimmediately adjacent the tip thereof.
 7. An eddy current testing circuitcomprising: a battery source of power; a common emitter transistorconnected to said battery; an iron core detector coil arranged to bedisposed in inductive relationship to metal to be tested; a capacitorconnected in parallel to a first portion of said detector coil through acenter tap to form a tuned circuit developing a normal testingfrequency; a direct collector-to-base feedback circuit connected betweensaid center tap of said coil and the base of the transistor andincluding, in series, a second capacitor and a second portion of saiddetector coil; means included in the base circuit of said transistor fordetermining with said second capacitor, the value of a blocking audiofrequency developed in said testing circuit, said means including: apotentiometer; and a trimming resistor connected in series with saidpotentiometer; an earphone connected in the emitter circuit of saidtransistor and responsive to said blocking audio frequency to emit anaudible signal indicative of changes in the inductive reactance of saidiron core detector coil; and a pulse shaping circuit comprising aresistor and capacitor connected in parallel between said earphone andthe emitter of said transistor in the emitter circuit of the transistorfor limiting current flow in said emitter circuit and shaping the pulsesof the blocking frequency developed by said blocking oscillator.